In order to make high data rate interactive services, such as video conference communication, more globally available there is a need for high speed data communications. One method of communication is called asymmetrical digital subscriber line (ADSL). Asymmetrical digital subscriber line (ADSL) is a transmission scheme that allows for the use of plain old telephone service (POTS) and a variety of digital channels on two-wire twisted metallic wire pair with mixed gauges. The facility of using twisted wire is in the availability of the existing infrastructure (i.e. the telephone lines), the dual use of which allows for reduced installation costs. For ADSL communications, technology standards have been adopted and are defined in the American National Standard for Telecommunication-Network and Customer Interfaces-Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, T1. 413-1995, ADSL coding standard, adopted Aug. 18, 1995.
One of the advantages of ADSL transmission is that it may be used to provide high quality, multiple and simultaneous interactive video services over an ordinary telephone line without disruption of the standard telephone service. The growing popularity of in-home entertainment, computation, and business coupled with the availability of existing infrastructure makes ADSL an important communication transmission method.
One method of modulation used in an ADSL communication system is the discrete multi-tone (DMT) modulation method. The DMT method transmits data on several frequency carriers, in contrast to more traditional single carrier modulation methods. DMT divides the available bandwidth into several smaller bands, each centered around one carrier frequency. Each carrier frequency is modulated with data such that each carrier frequency communicates data parallel in time. In other words, each modulated carrier is added together and the resulting frame is transmitted on the same physical line.
DMT modulation is especially useful for transmitting data on telephone lines, where line attenuation and noise characteristics may vary drastically as a function of frequency. Since DMT is essentially made up of several independent carriers, carriers operating at those frequencies having the greatest signal-to-noise ratio (SNR) carry many bits of transmission information, while carriers suffering greater attenuation and/or noise carry very few or no bits of transmission information. The DMT method allows the full capacity of the channel to be realized in an operable manner.
One of the requirements of ADSL transmission is that the receiver performs a synchronization to the incoming frames. A receiver typically is required to determine the point at which one frame ends and the next frame begins. Frame synchronization is defined as aligning the receiver's frame boundary optimally to the incoming data frame. There are many methods of performing synchronization incorporating a variety of algorithms and circuitry.
Specifically in DMT transmission, each frame is sampled several times by the receiver, often equaling twice the number of carriers. Typically, a phase lock loop (PLL) will lock the sampling clock optimally to the received data. The problem arises in determining which received sample is to be counted as the first sample in a frame. The starting point of a frame is unknown to the receiver and must be determined. Correlating the received data with the known transmitted data is one method of determining an optimal frame alignment. There are a variety of methods, but many synchronization solutions are costly and time consuming. The problem exists in ADSL communication systems of performing both inter-symbol interference reduction and frame alignment in a cost efficient, timely manner.